Specify a C compiler to satisfy new SFML requirement #516

Job	Run time
clang-format	7s
Windows MSVC Shared Release	3m 12s
Windows MSVC Shared Debug	2m 35s
Windows MSVC Static Release	3m 30s
Windows MSVC Static Debug	3m 35s
Windows ClangCL Shared Release	2m 59s
Windows ClangCL Shared Debug	3m 13s
Windows ClangCL Static Release	4m 8s
Windows ClangCL Static Debug	3m 51s
Windows Clang Shared Release	1m 24s
Windows Clang Shared Debug	2m 9s
Windows Clang Static Release	3m 20s
Windows Clang Static Debug	2m 40s
Linux GCC Shared Release	54s
Linux GCC Shared Debug	47s
Linux GCC Static Release	1m 24s
Linux GCC Static Debug	59s
Linux Clang Shared Release	1m 7s
Linux Clang Shared Debug	42s
Linux Clang Static Release	1m 18s
Linux Clang Static Debug	1m 52s
macOS Clang Shared Release	1m 34s
macOS Clang Shared Debug	1m 3s
macOS Clang Static Release	1m 27s
macOS Clang Static Debug	1m 31s
	51m 21s

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